Environmental concerns have resulted in legislation which places limits on the sulfur content of gasoline. In the European Union, for instance, a maximum sulfur level of 150 ppm by the year 2000 has been stipulated, with a further reduction to a maximum of 50 ppm by the year 2005. Sulfur in the gasoline is a direct contributor of SOx emissions, and it also poisons the low temperature activity of automotive catalytic converters. When considering the effects of changes in fuel composition on emissions, lowering the level of sulfur has the largest potential for combined reduction in hydrocarbon, CO and NOx emissions.
Gasoline comprises a mixture of products from several process units, but the major source of sulfur in the gasoline pool is fluid catalytic cracking (FCC) naphtha which usually contributes between a third and a half of the total amount of the gasoline pool. Thus, effective sulfur reduction is most efficient when focusing attention on FCC naphtha.
A number of solutions have been suggested to reduce sulfur in gasoline, but none of them have proven to be ideal. Since sulfur in the FCC feed is the prime contributor of sulfur level in FCC naphtha, an obvious approach is hydrotreating the feed. While hydrotreating allows the sulfur content in gasoline to be reduced to any desired level, installing or adding the necessary hydrotreating capacity requires a substantial capital expenditure and increased operating costs. Further, olefin and naphthene compounds are susceptible to hydrogenation during hydrotreating. This leads to a significant loss in octane number. Hydrotreating the FCC naphtha is also problematic since the high olefin content is again prone to hydrogenation.
Little has been reported on the selective permeation of sulfur containing compounds using a membrane separation process. For example, U.S. Pat. No. 5,396,019 (Sartori et al.) teaches the use of crosslinked fluorinated polyolefin membranes for aromatics/saturates separation. Example 7 of this patent reports thiophene at a level of 500 ppm.
U.S. Pat. No. 5,643,442 (Sweet et al.) teaches the lowering of sulfur content from a hydrotreated distillate effluent feed using a membrane separation process. The preferred membrane is a polyester-imide membrane operated under pervaporation conditions.
U.S. Pat. No. 4,962,271 (Black et al.) teaches the selective separation of multi-ring aromatic hydrocarbons from lube oil distillates by perstraction using a polyurea/urethane membrane. The Examples discuss benzothiophenes analysis for separated fractions.
U.S. Pat. No. 5,635,055 (Sweet et al.) discloses a method for increasing the yields of gasoline and light olefins from a liquid hydrocarbonaceous feed stream boiling in the ranges of 650° F. to about 1050° F. The method involves thermal or catalytic cracking the feed, passing the cracked feed through an aromatic separation zone containing a polyester-imide membrane to separate aromatic/non-aromatic rich fractions, and thereafter, treating the non-aromatic rich fraction to further cracking processing. A sulfur enrichment factor of less than 1.4 was achieved in the permeate.
U.S. Pat. No. 5,005,632 (Schucker) discloses a method of separating mixtures of aromatics and non-aromatics into aromatic enriched streams and non-aromatics-enriched streams using one side of a poly-urea/urethane membrane.
It would be highly desirable to use a selective membrane separation technique for the reduction of sulfur in hydrocarbon streams, in particular, naphtha streams. Membrane processing offers a number of potential advantages over conventional sulfur removal processes, including greater selectivity, lower operating costs, easily scaled operations, adaptability to changes in process streams and simple control schemes.